The production of polymer particles, such as polyethylene and polypropylene, by polymerising the respective monomer, and optionally a comonomer, in the gas phase is well known. In a fluidised bed polymerisation process, monomer and catalyst are passed into a reaction zone wherein they react to form polymer particles which are maintained in a fluidised state by continuously passing through the bed of polymer a gas, known as a fluidising gas. The fluidising gas usually comprises the monomer to be reacted, and optionally comonomers, as well as a number of other components well known to the person skilled in the art. Reactants and catalyst are introduced into the reaction zone to replace those that have reacted and produced polymer is withdrawn.
Fluidising gas which has passed through the bed of polymer particles is withdrawn from the reaction zone and recycled via an external conduit for re-use. The recycle gas comprises unreacted monomers and usually fresh monomer is added to this stream prior to the reaction zone to replace that which has reacted.
The polymerisation reaction itself is highly exothermic. In order to control the temperature of the reaction zone it is known to cool the recycling fluidising gas. It is also known to cool the recycling fluidising gas so that a portion of it condenses and forms a liquid, which can also be recycled to the reaction zone. Vaporisation of this liquid occurs providing significant cooling to the reaction zone.
Examples of such processes can be found in EP 824117, WO 97/25355 and WO 99/00430, each of which describe a polymerisation process where the recycling fluidising gas is cooled so that a portion of it condenses and forms a liquid, which liquid is then recycled to the reaction zone.
In a “balanced” reaction the rate of heat generation is balanced by cooling to maintain a constant temperature in the reaction zone. In practise, it is necessary to monitor the temperature of the reaction zone and have a system to compensate if the temperature starts to vary from that desired.
In particular, reaction rate increases can lead to an increase in the temperature of the reaction zone. The temperature increase can itself lead to a further increase in reaction rate, which due to the reaction exotherm can cause yet a further increase in temperature. The increase in temperature can result in production of off-specification material and large increases in temperature and reaction rate can lead to reaction well outside of the desired operating range, which can lead to fouling of the reactor and necessitate shut-down.
Any control system should allow the process to react to temperature changes quickly enough to keep the reaction temperature within well-defined limits (for a particular product). Thus, in a conventional system an increase in the temperature in the reactor above a desired temperature is compensated for by increasing the cooling applied to the recycle stream. In a system with condensation of a portion of the recycle stream this results in an increased amount of condensed liquid formed, which is then passed to the reactor where it effects additional cooling and cools the reactor again. Obviously the reverse applies if a decrease in the temperature in the reactor below a desired temperature is observed.
The time which the system takes to respond to the temperature increase or decrease may be referred to as the “deadtime” of the control scheme.
To date such control has been relatively straightforward because the time over which changes in temperature might appear and take effect has been relatively long compared to deadtime of typical control schemes.